Proteolytic cleavage of the precursor of von Willebrand factor is not essential for multimer formation

Monkey kidney cells (COS-1), transfected with full-length human von Willebrand factor (vWF) cDNA encoding the precursor of vWF (pro-vWF), mimic the characteristics of the biosynthesis and of the constitutive secretory pathway, displayed by cultured vascular endothelial cells. Such heterologous transfected cells are able to cleave pro-vWF, generating the propolypeptide and mature vWF, and to assemble pro-vWF dimers into a series of multimers, similarly to endothelial cells. Evidence is presented showing that proteolytic processing of pro-vWF by COS-1 cells occurs at the peptide bond between arginine and serine in the sequence Lys762-Arg763-Ser764, identical to endothelial cell-associated proteolysis. This conclusion stems from the observation that substitution of Arg763 by a glycine residue completely abolishes proteolytic processing. As a result, transfection of COS-1 with the mutant vWF-Gly763 cDNA does not significantly affect the multimeric organization of secreted vWF molecules. Consequently, we conclude that proteolytic processing of pro-vWF is not required for multimer formation. Pulse-chase labeling of COS-1 cells transfected with full-length vWF cDNA reveals pro-vWF exclusively in cell lysates, whereas both pro-vWF and mature vWF are encountered in the conditioned medium. These observations indicate that proteolytic processing of pro-vWF is a "late" event during intracellular routing of these molecules or may occur extracellularly.